Considering that Warner Brothers is aiming to have the next Superman film out by summer 2012 (a solid 4 years away), there are those wondering if keeping Routh is a bit premature. Normally, I would agree with that logic.
However if you will all remember, boys and girls. According to a script review from two months back, the character of Clark Kent makes an appearance in Green Lantern(!)
Furthermore, we know for a fact that Warner Brothers and DC Comics are trying to play the same card game that Marvel Films has (so far) successfully pulled off - i.e. set the franchises in the same universe/continuity so the various characters can interact with one another in the form of cameos, references, etc.
Knowing that and with the recent of news of Routh possibly staying on as the last son of Krypton, there's a solid chance that he could in fact done the glasses again in Green Lantern.
If for no other reason than to possibly set up the Superman reboot - which we've heard Warner Brothers intends to keep Superman: The Man of Steel as its title.